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1. NuSTAR Detection of X-Rays Concurrent with Gamma-Rays in the Nova V5855 Sgr

   Nelson, Thomas; Mukai, Koji; Li, Kwan-Lok; Vurm, Indrek; Metzger, Brian D.; Chomiuk, Laura; Sokoloski, J. L.; Linford, Justin D.; Bohlsen, Terry; Luckas, Paul (February 2019, The Astrophysical journal)

   We report the first detection of hard (>10 keV) X-ray emission simultaneous with gamma-rays in a nova eruption. Observations of the nova V5855 Sgr carried out with the NuSTAR satellite on Day 12 of the eruption revealed faint, highly absorbed thermal X-rays. The extreme equivalent hydrogen column density toward the X-ray emitting region (˜3 × 10^24 cm^-2) indicates that the shock producing the X-rays was deeply embedded within the nova ejecta. The slope of the X-ray spectrum favors a thermal origin for the bulk of the emission, and the constraints of the temperature in the shocked region suggest a shock velocity compatible with the ejecta velocities inferred from optical spectroscopy. While we do not claim the detection of nonthermal X-rays, the data do not allow us to rule out an additional, fainter component dominating at energies above 20 keV, for which we obtained upper limits. The inferred luminosity of the thermal X-rays is too low to be consistent with the gamma-ray luminosities if both are powered by the same shock under standard assumptions regarding the efficiency of nonthermal particle acceleration and the temperature distribution of the shocked gas. 

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2. On the Anisotropy of Galactic Cosmic Rays

   https://doi.org/10.3847/1538-4357/ab24c1  

   Schlickeiser, R.; Oppotsch, J.; Zhang, M.; Pogorelov, N. V. (July 2019, The Astrophysical Journal)
3. X-Rays Trace the Volatile Content of Interstellar Objects

   https://doi.org/10.3847/1538-4357/ace970  

   Cabot, Samuel H.; Wang, Q. Daniel; Seligman, Darryl Z. (October 2023, The Astrophysical Journal)

   Abstract The nondetection of a coma surrounding 1I/‘Oumuamua, the first discovered interstellar object (ISO), has prompted a variety of hypotheses to explain its nongravitational acceleration. Given that forthcoming surveys are poised to identify analogs of this enigmatic object, it is prudent to devise alternative approaches to characterization. In this study, we posit X-ray spectroscopy as a surprisingly effective probe of volatile ISO compositions. Heavily ionized metals in the solar wind interact with outgassed neutrals and emit high-energy photons in a process known as charge exchange, and charge-exchange-induced X-rays from comets and planetary bodies have been observed extensively in our solar system. We develop a model to predict the X-ray flux of an ISO based on its chemical inventory and ephemeris. We find that while standard cometary constituents, such as H₂O, CO₂, CO, and dust, are best probed via optical or infrared observations, we predict strong X-ray emission generated by charge exchange with extended comae of H₂and N₂—species that lack strong infrared fluorescence transitions. We find that XMM-Newton would have been sensitive to charge exchange emission from 1I/‘Oumuamua during the object’s close approach to Earth, and that constraints on composition may have been feasible. We argue for follow-up X-ray observations of newly discovered ISOs with close-in perihelia. Compositional constraints on the general ISO population could reconcile the apparently self-conflicting nature of 1I/‘Oumuamua and provide insight into the earliest stages of planet formation in extrasolar systems. 

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4. The Impact of Cosmic Rays on the Kinematics of the Circumgalactic Medium

   Butsky, Iryna S.; Werk, Jessica K.; Tchernyshyov, Kirill; Fielding, Drummond B.; Breneman, Joseph; Piacitelli, Daniel; Quinn, Thomas R.; Sanchez, N. Nicole; Cruz, Akaxia; Hummels, Cameron B.; et al (January 2022, The Astrophysical journal)

   We use hydrodynamical simulations of two Milky Way-mass galaxies to demonstrate the impact of cosmic-ray pressure on the kinematics of cool and warm circumgalactic gas. Consistent with previous studies, we find that cosmic-ray pressure can dominate over thermal pressure in the inner 50 kpc of the circumgalactic medium (CGM), creating an overall cooler CGM than that of similar galaxy simulations run without cosmic rays. We generate synthetic sightlines of the simulated galaxies' CGM and use Voigt profile fitting methods to extract ion column densities, Doppler-b parameters, and velocity centroids of individual absorbers. We directly compare these synthetic spectral line fits with HST/COS CGM absorption-line data analyses, which tend to show that metallic species with a wide range of ionization potential energies are often kinematically aligned. Compared to the Milky-Way simulation run without cosmic rays, the presence of cosmic-ray pressure in the inner CGM creates narrower OVI absorption features and broader SiIII absorption features, a quality which is more consistent with observational data. Additionally, because the cool gas is buoyant due to nonthermal cosmic-ray pressure support, the velocity centroids of both cool and warm gas tend to align in the simulated Milky Way with feedback from cosmic rays. Our study demonstrates that detailed, direct comparisons between simulations and observations, focused on gas kinematics, have the potential to reveal the dominant physical mechanisms that shape the CGM. 

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5. The Impact of Cosmic Rays on the Kinematics of the Circumgalactic Medium

   https://doi.org/10.3847/1538-4357/ac7ebd  

   Butsky, Iryna S.; Werk, Jessica K.; Tchernyshyov, Kirill; Fielding, Drummond B.; Breneman, Joseph; Piacitelli, Daniel R.; Quinn, Thomas R.; Sanchez, N. Nicole; Cruz, Akaxia; Hummels, Cameron B.; et al (August 2022, The Astrophysical Journal)

   Abstract We use hydrodynamical simulations of two Milky Way–mass galaxies to demonstrate the impact of cosmic-ray pressure on the kinematics of cool and warm circumgalactic gas. Consistent with previous studies, we find that cosmic-ray pressure can dominate over thermal pressure in the inner 50 kpc of the circumgalactic medium (CGM), creating an overall cooler CGM than that of similar galaxy simulations run without cosmic rays. We generate synthetic sight lines of the simulated galaxies’ CGM and use Voigt profile-fitting methods to extract ion column densities, Doppler-bparameters, and velocity centroids of individual absorbers. We directly compare these synthetic spectral line fits with HST/COS CGM absorption-line data analyses, which tend to show that metallic species with a wide range of ionization potential energies are often kinematically aligned. Compared to the Milky Way simulation run without cosmic rays, the presence of cosmic-ray pressure in the inner CGM creates narrower Oviabsorption features and broader Siiiiabsorption features, a quality that is more consistent with observational data. Additionally, because the cool gas is buoyant due to nonthermal cosmic-ray pressure support, the velocity centroids of both cool and warm gas tend to align in the simulated Milky Way with feedback from cosmic rays. Our study demonstrates that detailed, direct comparisons between simulations and observations, focused on gas kinematics, have the potential to reveal the dominant physical mechanisms that shape the CGM. 

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